Flue or vent gases from the combustion of fuels such as coal or residual oil, the pyrolysis of solid waste, and the venting of chemical processes has been a matter of increasing concern for many years. Components such as sulfur dioxide and nitrogen oxides (NO.sub.x) are accused of fostering acid rain, having toxic effect on human, lower animal and plant life, and being generally dangerous or obnoxious even in quite low concentrations.
As a result, many approaches have been proposed to cleanse the waste gases of their toxic components. Scrubbing with a compound in slurry form with water has been by far the most popular approach. Wet scrubbing systems, although effective in removing toxic gases, have been very expensive and highly corrosive and have often simply converted an air-pollution problem into a sludge-disposal or water-pollution problem. Even with wet scrubbing processes involving reactants that are regenerable, thus producing no waste sludge, problems have occurred. In the magnesium oxide wet scrubbing process, for example, major technical difficulties have been encountered in handling the corrosive liquid slurries, and large energy costs have been incurred in drying the slurries and in regenerating the magnesium oxide.
In recent years several so-called dry sorbent processes have been proposed. In these processes particulate reactant is brought into direct contact with the undesirable components of a waste gas stream where it reacts with those components in a dry or semi-dry state. Principal advantages of these processes have been their simplicity, decreased mechanical problems, and generally lower costs. A major disadvantage has been their low cleansing efficiencies; at best generally only 50 to 60 percent of the toxic components are removed.
Various approaches have been examined in an attempt to increase the reactivity and cleansing efficiency of particulate reactants. One such approach involves the use of silicate particles having a huge surface area, e.g., asbestos, exfoliated vermiculite, or expanded (popped) perlite.
Thus, U.S. Pat. No. 4,061,476 teaches that expanded perlite optionally is added to a solid absorbent. Pulverulent amorphous silicon dioxide, charcoal, carbon dust, lime hydrate, bog iron ore, pulverulent iron oxide, and dolomite dust are taught as being useful solids called variously "absorption" or "sorption" agents or "absorbents". Popped perlite is stated to have good physical absorption properties and is suitable for supplying water to the reaction; alternatively the gas in process can be humidified.
U.S. Pat. No. 4,201,751 to the same inventors teaches use of exfoliated vermiculite and expanded perlite, suitably moistened with water and caustic soda, as a carrier for a sprayed-on aqueous suspension of absorption agent. Such agent is rich in ferric oxide-containing steel mill dust; higher alkalinity is imparted to the suspension by dispersing calcium hydroxide and/or calcium oxide therein; the further mixing in of bog iron ore and/or a heavy metal salt such as one of lead or copper is stated to improve the sorption of various acidic gases. Lime-alkalized steel mill dust on an expanded perlite carrier is exemplified for treating contaminated gas. When the absorption agent becomes exhausted by contact with SO.sub.2, it can be roasted to drive off SO.sub.2 with preservation of the perlite. Perlite does not tend to sinter until above 980.degree. C.
An earlier patent, U.S. Pat. No. 3,882,221, appears to prefer aqueous caustic soda or potash solutions coated onto exfoliated vermiculite or expanded perlite for gas cleaning purposes. However, this patent exemplifies wetting one or the other of these two expanded minerals with an aqueous dispersion of calcium oxide and of calcium hydroxide to coat such carrier, then reacting the coatings with chlorine and carbon dioxide gases, respectively. Details of the preparation and operating efficiency of these chalky coatings are not evident from the text, and their effectiveness for removing SO.sub.2 and NO.sub.x only can be speculated on. The reaction products obtained are stated as being capable of being removed from the expanded mineral with water or other unnamed suitable solvent.
It has now been discovered that exfoliated or expanded minerals, such as vermiculite or perlite, freshly coated with a hydrated divalent metal oxide, can be conditioned in a simple manner to produce an improved acceptor for cleansing waste gases at temperatures below 250.degree. C. Complete and efficient removal of mixed SO.sub.2 and NO.sub.2 has been obtained consistently when using gases with contaminant concentrations representative of those in waste gases, such as a flue gas from the combustion of coal. Furthermore, an improved coating method has been devised for obtaining very high, waste-free loading of MgO and CaO on the exfoliated or expanded mineral for extended sorption service. The conditioning method has been found to result in an interaction between the MgO or CaO and the mineral carrier that results in a strong bond between the phases. Also, the conditioning has been found to increase markedly the strength of the carrier, resulting in significantly less degradation in handling. Additionally, the magnesium oxide embodiment of the improved acceptor has been found, surprisingly, to be repeatedly regenerable without noticeable degradation at a much lower temperature than related prior art acceptors, and that regeneration can be practiced efficiently in a manner that allows the simultaneous removal and separate collection or destruction of SO.sub.2 and NO.sub.x when these gases have been sorbed together.